1. Field of the Invention
The invention relates to a hydraulic control system for controlling the operation of a variable camshaft timing (VCT) system. More specifically, the present invention relates to a control system that utilizes a position sensor mounted to a differential pressure control system (DPCS) with a centrally mounted spool valve and a control loop controlling the position of the spool valve.
2. Description of Related Art
U.S. Pat. No. 5,107,804 shows one method of how to control the position of a spool valve that controls the oil flow to and from the chambers of a vane or piston style cam phaser by using an externally mounted solenoid Differential Pressure Control System (DPCS). The DPCS utilizes engine oil pressure to push against one end of a spool valve. A control pressure pushes against the other side and comes from either a PWM or a proportional solenoid.
The control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of the spool valve. U.S. Pat. No. 5,184,578 shows the control system, in which crank and cam positions are sensed and a Pulse-Width Modulated Solenoid moves a spool valve to control the actuation of the phaser, with a closed-loop control measuring the phase difference between cam and crank and the operating spool valve accordingly.
FIG. 1 shows a block diagram of a cam torque actuated variable cam timing device with a differential pressure control system (DPCS). The Engine Control Unit (ECU) (1) decides on a phase set point (2) based on various demands on the engine and system parameters (temperature, throttle position, oil pressure, engine speed, etc. . . . ). The set point is filtered (3) and combined (4) with a VCT phase measurement (12) in a control loop with a PI controller (5), phase compensator (6), and anti-windup logic (7). The output of this loop is combined (9) with a null duty cycle signal (8) into a Pulse Width Modulated (PWM) valve (206) that provides physical pressure (340) to the Differential Pressure Control System (DPCS) (234), along with oil pressure from the main oil gallery or supply (230) to push the center mounted spool valve. The spool valve (192), in turn, controls fluid (engine oil) to activate the VCT phaser (14), either by applying oil pressure to the vane chambers or by switching passages to allow cam torque pulses to move the phaser (14). The cam position is sensed by a cam sensor (20), and the crank position (or the position of the phaser drive sprocket, which is connected to the crankshaft) is also sensed by the sensor (21), and the difference between the two is used by a VCT phase measurement circuit (19) to derive a VCT phase signal (12), which is fed back to complete the loop.
One problem with this system is that the pulse width modulated (PWM) valve and the DPCS (234) with the center mounted spool valve have both frictional and magnetic hysteresis. As the duty cycle (320) or pulse width modulated signal increases and enters the pulse width modulator (206), the physical pressure (340) that results will be different then when the duty cycle (320) decreases, creating frictional hysteresis, as shown in graph (360) of FIG. 1. The pressure (340) then feeds into the DPCS (234), which determines the position of the center mounted spool valve (192). As a result, as the duty cycle (320) increases or decreases, different positions of the spool valve (192) results, creating magnetic hysteresis, as shown in graph (370) of FIG. 1.
Therefore, there is a need in the art for a system, which minimizes errors due to frictional and magnetic hysteresis.
The cam phaser of the present invention includes a differential pressure control system (DPCS) with spool position feedback to control the position of a center mounted spool valve and control the phase angle of the cam mounted phaser. A position sensor is mounted to the spool valve position such that a control loop controls the position of the spool valve. A second outer loop controls the phase angle. An offset is preferably added to the spool valve position to move the spool valve to its steady state or null position. This null position is required so that the spool can move in to move the phaser in one direction and move out to move the phaser in the other direction. This type of system reduces any frictional or magnetic hysteresis in the system.